"Scholars have for decades sought to measure judicial ideology and its impact on judicial behavior. They have not, however, taken care to identify the phenomenon that they are measuring in clear and convincing terms, or to evaluate different measurement approaches for their suitability to the task at hand. In this paper, we diagnose recurring weaknesses of the existing empirical literature, and we propose a measurement approach of our own that offers significant improvements over some of the most popular alternatives.

"First, we identify the major conceptual and methodological obstacles to the empirical measurement of judicial ideology. Foremost among the conceptual challenges to be faced are the difficulty of defining ideology and the associated challenge of disentangling ideological and nonideological preferences. The failure of empirical researchers to confront this challenge is a fatal one because it is impossible to design and select appropriate tools for the measurement of a phenomenon that has not been properly specified.... Second, we discuss the strengths and weaknesses of different measurement approaches, and their suitability to different types of research questions.... Third, we perform a head-to-head competitive evaluation of two popular measurement approaches and a third approach of our own devising.

"We test the predictive power of our approach against that of two popular proxy measures of judicial ideology-namely, party of appointing president and judicial "common space scores." For testing purposes, we use a data set that we coded consisting of approximately 1,900 asylum cases decided by the Ninth Circuit over a ten-year period. We find that common space scores perform only slightly better than party of appointing president at predicting actual judicial voting. Moreover, both proxy measures yield almost identical estimates of the impact of ideology on actual judicial voting."